Acid and a metal carbonate

The reaction between an acid and a metal carbonate is easy to recognise because it fizzes and the gas turns limewater milky. It is used to prepare salts, to test for carbon dioxide and to identify a carbonate, so it is worth a secure equation in 4541.

The balanced equation

The general word equation is: metal carbonate + acid → salt + water + carbon dioxide. With correct formulae and state symbols, common examples are:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

CuCO₃(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) + CO₂(g)

For an insoluble carbonate reacting with acid, the ionic equation keeps the solid and shows the acid supplying hydrogen ions:

CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g)

For a soluble carbonate such as sodium carbonate, the carbonate ion is free in solution and the ionic equation is CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g). Balancing carbon, hydrogen and oxygen across three products is where care is needed.

Conditions required

No heat and no catalyst are required; the reaction happens on mixing at room temperature with a dilute acid. Any acid works, hydrochloric, sulfuric or nitric, because the carbonate is not a metal and no hydrogen is produced. When the carbonate is insoluble, an excess of it is added to a warm acid and the leftover solid is filtered off; this is a standard way to prepare a soluble salt.

What you observe

You see brisk effervescence as the carbonate reacts, and the solid dissolves if it is soluble in the resulting salt solution. The gas given off is carbon dioxide. The confirming test is to bubble the gas through limewater (calcium hydroxide solution): the limewater turns milky, because insoluble calcium carbonate forms. Colour depends on the carbonate, green copper(II) carbonate gives a blue copper(II) salt solution, while calcium and sodium carbonates give colourless solutions. The reaction is exothermic, so the mixture warms slightly.

Where it appears in the SPM exam

In 4541/1 it is tested as the limewater test for carbon dioxide and as a way to tell a carbonate apart from a metal. In 4541/2 it supports salt-preparation questions (insoluble-base or excess-solid method) and equation writing, including the ionic equation. In the practical paper 4541/3, marble chips (calcium carbonate) reacting with hydrochloric acid is a favourite rate-of-reaction experiment, measured by loss of mass or volume of carbon dioxide collected against time.

How we teach it

We focus on three habits: balancing all three products, remembering the limewater test with its reason (milkiness from calcium carbonate), and choosing an excess of an insoluble carbonate when the question asks you to prepare a salt. The most common error is leaving out the water or writing the gas as CO instead of CO₂. Tying the observation to the equation makes both the calculation and the identification questions straightforward, and it prepares you for the rate experiment that so often follows.

Quick summary

To answer with confidence, follow one routine: identify the carbonate and the acid, name the salt from the acid anion and the metal cation, then balance the water and the carbon dioxide. For zinc carbonate with dilute sulfuric acid this gives ZnCO₃(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) + CO₂(g). Remember that the tell-tale evidence is effervescence plus limewater turning milky, and that the same gas and the same test appear whether the carbonate is calcium, copper, zinc or sodium. Getting this pattern secure means you can handle a salt preparation, an ionic equation and a gas-identification question from one piece of understanding.